Monday, February 1, 2010
BGP MED Configuration
When using BGP, sometimes the Autonomous System (AS) wants to influence another AS's routing decision on how incoming packets are routed to its AS. Multiexit-discriminator (MED) is used to configure this setting for BGP peers. Unfortunately MED is one of the last considered processes in the BGP routing process. That means that if local preference is used in the BGP process it can not knowingly override any MED settings you are sending to that remote AS.
Route maps can be used with MED to better load balance settings, using the command default-metric under the BGP configuration will set MED settings. However if two BGP peers in the same AS have different MED values, the lower cost will always be preferred. While the other peer will receive little to almost no routing traffic besides BGP updates. What would be better is creating a route map that specifies which networks should have lower metrics on each BGP peer as shown in the example above.